1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a lamp driving apparatus for preventing damages to a lamp caused by electric current variation due to characteristics of the lamp.
2. Description of the Related Art
In general, the liquid crystal display (hereinafter, referred to as “LCD”) is broadly classified into reflection type LCD and transmission type LCD. A reflection type LCD device displays an image by reflecting the light externally provided to an LCD panel, and a transmission type LCD device displays an image with the light generated from a backlight assembly disposed under an LCD panel.
The backlight assembly for a LCD device includes a lamp unit which emits light, a light guide plate which guides the light emitted from the lamp unit toward an LCD panel, an optical sheet such as a diffusion sheet, and prism sheet or the like which increases brightness of the light from the light guide plate.
The lamp unit includes a lamp used as a light source of the LCD device, a reflection plate that reflects the light from the lamp to increase the light efficiency of the light emitted from the lamp, and an inverter electrically connected to the lamp to apply power thereto.
Recently, LCD devices that are lighter and smaller than a CRT (cathode ray tube) type display apparatus have been developed. Such LCD devices also have functions such as full color and high resolution. Owing to such advantages, the liquid crystal display devices are widely used for computer monitors, television receivers, and other display devices.
In the LCD technology, the brightness for an LCD device is important in terms of the display quality and efficiency. Particularly, the LCD devices require multiple lamps due to enlargement of the LCD panel. In general, a large size LCD device employs a plurality of lamps disposed under a diffusion sheet, which emit the light directly illuminated onto an LCD panel.
In an edge type LCD device of which a plurality of lamps are disposed adjacent to side portions of a light guide plate, the plurality of lamps, that is, at least two lamps, are disposed at the side portions of the light guide plate in order to provide the light enough to display images on the large size LCD.
However, a plurality of lamps disposed at the side portions of a light guide plate are connected to one transformer installed in an inverter in parallel with the transformer. So, electric current variation occurs between the lamps due to characteristics of each of the lamps and a circuit part that controls only the sum of electric current flowing through the plurality of lamps.
In a case where the electric current is not uniformly applied to each of the lamps, one or more lamps receiving relatively larger amount of electric current are damaged. As a result, the damaged lamp(s) among the plurality of lamps arranged in parallel with each other is shut down.
In order to increase the brightness of an LCD panel, each of the lamps is set and driven with electric current having a level slightly lower than that of the maximum allowable electric current. Thus, when the electric current flowing through a lamp exceeds the maximum allowable electric current, the lifetime of the lamp that receives the excessive electric current is decreased rapidly.
Also, a lamp that receives relatively larger amount of electric current becomes rapidly weakened even though the electric current does not exceed the maximum allowable electric current.
Moreover, in a case where at least three lamps are disposed at opposite side portions of the light guide plate, since the distance between each of the three lamps and a reflection plate of the lamp is different from each other, the leakage current and the electric current variations of the three lamps occur.